Various cutting or grinding oils are generally used to improve the productivity of machining which extends the life of tools for machining such as drills, endmills, turning tools and grinding wheels, and improves the roughness of machined surface and the machining efficiency thereby.
Oils for cutting and grinding are roughly classified into two types; water-soluble ones which are used after diluting the surface-active agent and lubricant component contained therein with water, and water-insoluble ones which contain a mineral oil as a main component and are used as it is. In conventional cutting and grinding operations, relatively large amounts of cutting or grinding oil are supplied to a spot of machining in the case of using either type.
The most basic and important functions of oils for cutting and grinding are lubricating and cooling function. Generally speaking, water-insoluble oils for cutting and grinding are superior in lubricity, while water-soluble ones are superior in cooling properties. Since the cooling properties of the water-insoluble oils for cutting and grinding are inferior to that of the water-soluble ones, it is necessary to supply a large amount of water-insoluble cutting or grinding oil ranging from several liters to several tens of liters per a minute.
From the different point of view, oils for cutting and grinding, which are effective for improving machining efficiency, have some drawbacks. The typical examples of such drawbacks are environmental problems. Whether the oil is water-soluble or water-insoluble, oil tends to be gradually deteriorated during the use thereof and finally becomes unusable for further use. For instance, in the case of water-soluble oil, the growth of microorganisms results in the separation of the components or the deterioration of the environmental sanitation due to the decrease of liquid stability and then the oil falls into disuse. In the case of water-insoluble oil, the acid components corrode the metallic parts or significantly change the viscosity of the oil due to the progress of oxidation, and then the oil also falls into disuse. Furthermore, the oil is consumed by attaching to metal chips and becomes wastes.
In such a case, the deteriorated oil is disposed and replaced with a fresh oil. The oil disposed as wastes requires various treatments so as not to adversely affect the environment. For instance, oils for cutting and grinding which have been developed under the priority to improve working efficiency use a large amount of chlorine-containing compounds, which may generate hazardous dioxin upon incineration. Therefore, the removal of such compounds is inevitable. Consequently, cutting or grinding oils comprising chlorine-free compounds have been developed. Even though the oils for cutting and grinding do not contain such hazardous compounds, environmental problems may arise due to a large amount of waste disposal. The water-soluble oils require complicated and expensive treatment of the oils to avoid the potential pollution of the surrounding water atmosphere.
Recently, in order to cope with the foregoing problems, it has been studied to cool a cutting or grinding portion by blowing a cold air instead of using cutting or grinding oils. In such a case, the other performance of oils for cutting and grinding, i.e., lubricity cannot be obtained.
Under such a background, there has been developed a method for cutting and grinding by a minimal quantity lubrication system. This system supplies minimal quantity oil ranging from about 1/100,000 to 1/1,000,000 of the normal quantity to a cutting or grinding spot of a workpiece together with a compressed fluid (for example, compressed air). This system can obtain a cooling effect with compressed air and can reduce the amount of wastes because of the minimization of oil use, which leads to improve the environmental influence caused by a large amount of waste disposal.